1. Field of the Invention
This specification relates to a switchgear, which is contactably installed at a ground electric power distribution line and has functions of line divergence, a circuit switching, and breaking a circuit between a line or an electric load side and an electric power source side when a fault current is detected on the line or load side, and particularly, to a switchgear for a ground distribution line, configured by insulating a conductor using epoxy or silicon as an insulating material and employing vacuum interrupters in each switch unit.
2. Background of the Invention
In general, an electric power generated in an electric power station is transmitted by a long-range transmission with ultrahigh voltage via a power transmission line and the ultrahigh transmission voltage is supplied to consumers via an electric power distribution system in a state of being stepped down to a distribution voltage (for example 22.9 kilo volts) at a substation. The electric power distribution system of electric power systems is classified into an overhead distribution system and a ground distribution system.
A switchgear for a ground distribution line is an apparatus for performing several functions, such as diverging a line, opening or closing an electric circuit, or breaking a system when detecting a fault current, on a ground line of a ground distribution system of the transmission and distribution electric power systems.
A switchgear applied to the electric power distribution system is connected to a plurality of divergent circuits on a three-phase high-pressure distribution line to break a fault current and switching the electric circuit, so a multi-circuit switchgear that more than three or four three-phase switches are mounted in one product is used. Accordingly, sulfur hexafluoride (SF6) gas is used for insulation between switches and insulation between phases. A tank as an enclosure is made to accommodate the switches therein and filled with the insulating gas to be sealed. A switching mechanism for switching switches into on position or off position is fixed to the outside of the tank. Also, in order to connect a conductor within the tank to an external cable, a bushing, which has a conductor therein and whose outside is molded by use of an insulating material, is used. The tank is made of steel. The tank is grounded to ensure user's safety. The related art switchgear for the ground distribution line having the structure uses the insulating gas for the insulation between phases, so it is also referred to as a gas-insulated switchgear.
The increase in electric power consumption derives a remarkable increase in use of the gas-insulated switchgear and SF6 gas accordingly. However, the SF6 gas has been pointed out as one of main factors causing global warming gas. The greenhouse effect on Earth caused due to the SF6 gas is 23,900 times more severe than carbon dioxide. Hence, use of the SF6 gas is under regulation or will be regulated around the world.
Hereinafter, description will be given of a typical configuration of a gas-insulated switchgear according to the related art.
FIG. 1 is a perspective view schematically showing a gas-insulated switchgear according to the related art, FIG. 2 is a perspective view that switch units and bus bars are connected to the gas-insulated switchgear of FIG. 1, and FIG. 3 is a view of a single switch unit of FIG. 1.
Referring to FIGS. 1 to 3, the related art gas-insulated switchgear includes a tank 1 containing SF6 gas therein and defining an enclosure, a switching mechanism 2 for driving the switchgear to one of three positions, namely, closing, opening or earthing (grounding) positions, a ground mold cone 3 serving as a safety element for earthing a current charged in a conductor of each switch unit (including a movable contactor and a stationary contactor for each phase, see FIG. 3) within the tank 1, an element 4 for transforming a voltage supplied from an electric power source side to the switchgear as a Direct Current power source for a controller, an insulating cable 5 for connection of a power source bushing of the enclosure, a common bus bar 6 for connecting respective contactors within the switch units for each of R, S and T phases.
The switchgear further includes bushings 7 disposed at an outer surface of the switchgear and connected to a cable heading to a user to maintain an insulation with the tank 1 and simultaneously maintain an air-tight state for prevention of gas leakage when electrically connecting the conductor within the tank 1 to an external conductor, and switch units 8 each having an arc-extinguishing capability for extinguishing arc generated upon opening or closing the electric power circuit and each having a movable contactor 9 and a main circuit stationary contactor 11 or a ground circuit stationary contactor 12. Referring to FIG. 3, the movable contactor 9 contacts the stationary contactor 11 or 12 (i.e., moving to a closing position or ground position) or is separated from the stationary contactor 11 or 12 (i.e., moving to an opening position), to thus transfer or stop transferring an income current through the common bus bar 6 to the load side. A reference numeral 10 denotes a bus bar for connection of the common bus bar 6.
In the structure of the switch unit of FIG. 3, the main circuit stationary contactor 11 is connected to the bushing 7, and the ground circuit stationary contactor 12 is connected to the ground bushing, namely, the mold cone 3. Here, when the movable contactor 9 is connected to the ground circuit stationary contactor 12, a current charged in the conductor is discharged to the earth.
The related art gas-insulated switchgear includes 3 or 4 switch units 8 for 3 or 4 phases arranged in the tank 1. The switch units 8, namely, the switch units 8 for three R, S, T phases are connected to the common bus bar 6, respectively, and a driving shaft for driving the movable contactor 9 of each switch unit 8 is connected to the switching mechanism 2, thereby operating (driving) the movable contactor 9 of each switch unit 8 to one of three positions, namely, closing, opening and earthing positions. The switching mechanism 2 for each switch unit 8 is attached on the outside of the tank 1, and the switchgear has a display for informing a position (closing, opening or earthing position) of the corresponding switch unit 8. The tank 1 is filled with SF6 gas, accordingly, arc generated when the switch unit 8 opens the load is extinguished and an insulation between conductors inside the tank 1 is maintained. Injection pressure of SF6 gas is 6 to 10 psi. An explosion proof plate is mounted at a lower side of the tank to prevent damages caused by explosion of the tank 1 due to high pressure generated when a fault current breaking occurs in the tank 1.
However, the related art gas-insulated switchgear has the following problems.
First, use of SF6 gas, which affects the global warming, will be restricted in the usage amount in future.
Second, the tank may be in danger of explosion due to an instantaneous pressure increase upon occurrence of a fault current breaking in the tank, which contains high-pressure gas.
Third, gas pressure affects insulation and arc-extinguishing capabilities, and accordingly maintenance for gas leakage is required, which results in requirements of manpower and costs.
Fourth, to fabricate the switch units, many constituting elements for constructing a conductive section, an insulating section and an arc-extinguishing section should be assembled, and such assembly requires excessive time.
Fifth, the structure that a plurality of circuits are air-tightly accommodated in one tank may make it difficult to repair and maintain the tank because the tank should be disassembled even for a locally simple repair and maintenance.
Sixth, the insulation between phases and the insulation between each phase-based switch unit and the earth within the tank completely depend on insulation capability of gas. Accordingly, upon occurrence of a ground fault or a shortage between phases, a local insulation problem within the tank may effect to the entire product, which may cause the product to be completely unusable.